1. Field of the Invention
This invention relates broadly to eye surgery. More particularly, this invention relates to refractive laser systems for eye surgery.
2. State of the Art
The laser refractive surgery (or laser keratectomy) field has exploded over the past few years with many new lasers and algorithms to correct human vision. Systems are now using laser wavelengths from the ultraviolet (excimer) to the infrared to change the shape of the cornea in a calculated pattern which makes it possible for the eye to focus properly. For example, in the treatment of myopia, the excimer laser is used to remove or ablate tissue from the cornea in order to flatten its shape. Infrared (IR) energy is also used by some companies to treat myopia by reshaping the corneal tissue by a xe2x80x9cthermalxe2x80x9d method as opposed to ablation with the excimer wavelength. The correction of hyperopia is produced by steepening the cornea by removing tissue at the outer edges of the cornea (excimer) or by reshaping the cornea at the outer edges (IR energy). The correction of astigmatism, both myopic and hyperopic, requires the laser to remove (as done by the excimer laser) or reshape (as done by the IR laser) tissue in a more complex pattern. Laser corneal reshaping procedures are effective for correcting impaired visual acuity, but many researchers now believe this effectiveness needs to be improved. The proper model and delivery of the laser energy for removing or altering the tissue has been a major discussion recently as postoperative studies are indicating that current procedures may actually be inducing aberrations in the eye optical system.
Initial systems approved by the FDA implement the refractive corrections by a broadbeam approach; i.e., by delivering beam-shaped laser energy based on thin lens theory and paraxial optics applied to a single spherical surface. The beam is shaped by a motorized iris (myopia and hyperopia) and motorized slit (astigmatism) based on profiles derived through Munnerlyn""s derivation (C. R. Munnerlyn, S. J. Koons, and J. Marshall, xe2x80x9cPhotorefractive keratectomy: a technique for laser refractive surgeryxe2x80x9d, J. Cataract Refract. Surg. 14, 46-52 (1988)). Systems using this approach are currently marketed by VISX and Summit. More than one million eyes have been treated in this manner in the United States. However, this approach is limited, as it symmetrically treats a broad area of the cornea all at one time. Eye topography maps and, more recently, wavefront analysis reveal that the cornea is a very complex structure with many minute variations across its surface. The broadbeam laser approach cannot correct these minute variations.
A more recent approach to laser keratectomy uses a scanning laser spot system in which a small laser spot (typically 0.5-mm to 1.0-mm in diameter) is scanned across the cornea in a predetermined pattern to achieve refractive corrections. These systems differ in that they are more flexible than the broadbeam approach. With the control of a small spot, different areas of the cornea can be shaped independently of other areas. The scanning spot approach, therefore, allows for the laser beam to be moved in a specific pattern over the cornea to more particularly correct the shape of the cornea. However, there are several problematic issues with scanning spot systems.
First is the issue of treatment time. Scanning spot systems require longer refractive surgery times. The scanning spot is a slower approach since the small laser spot has to be moved over a wide surface (up to 10-mm for hyperopia). The scanning spot system typically delivers several hundred spots per treatment layer, and consequently treatment times are generally long. The broadbeam approach is much quicker as the entire cornea is treated with each laser pulse, or treatment layer.
Second is the issue of safety. The broadbeam laser is inherently safe from a treatment interruption standpoint because the cornea is treated symmetrically for each pulse; the iris represents a circle and the slit represents a rectangle so that every point on the cornea being treated is treated the same with each laser pulse. If the procedure is interrupted, there will always be some symmetrical spherical correction which can be continued more easily at a later time. However, the scanning spot, with its small spot size, cannot cover the entire corneal surface with one laser pulse. Thus, if an interruption occurs, there is no guarantee of a complete corneal etch for a layer at the point of interruption. Continuation at the point of interruption would be difficult.
Third is the issue of tracking. In the scanning spot system the eye needs to be tracked very carefully in order to deliver the spot to the correct point on the cornea as the eye moves. This is not as much of a problem in the broadbeam system as a broader area is treated with each pulse.
Fourth is the issue of surface roughness. The necessary overlap of laser spots tends to create roughness in the resulting etch. While it is necessary to overlap spots to provide complete coverage for a given ablation zone, regions of overlap will be ablated at twice the etch depth per pulse. The smoothness of the ablated volume is dependent on the spot overlap and to a lesser extent, the ratio of spot diameter and ablation zone diameter. This problem is not seen in the broadbeam approach.
Current FDA-approved refractive laser systems do not directly use eye-modeling systems, such as corneal topographers or wavefront sensors, to create an individual treatment profile for a patient""s eye. Rather, a topographic map is used indirectly by the surgeon in optimizing the treatment plan (diopter correction and astigmatic axis). More recently, eye contour topography has been used to more accurately provide refractive correction. There are systems currently going through FDA trials that do use corneal topographic surface data to directly guide the laser treatment algorithm. This is accomplished using a scanning laser spot, as current broadbeam laser refractive surgery systems cannot provide the laser beam detail required to use the topographic map data. Using the scanning spot system, each eye is individually analyzed as to its contour before ablation is applied. The goal with these systems is to take into account the varying degrees of curvature and height variations across the corneal surface, as opposed to assuming a perfect spherical surface as is currently done in broadbeam systems. Once these curvatures and height variations are determined by eye topography, they can be integrated with the refraction correction derivation of Munnerlyn to create customized ablation patterns for each individual eye.
The corneal topography approach has a drawback in that only measurements of the cornea are used. However, the eye is a complex optical system of which the cornea is only one component. Thus, even corneal topography information when combined with the current FDA-approved refraction equation, is not capable of suggesting what correction must be made to the corneal shape in order to optimally correct the overall aberration of the eye""s optical system.
There have been several recent approaches to the above problems. First, by expanding the mathematical equations for refraction correction to include higher order effects, coma (3rd order) and spherical (4th order) aberrations can be reduced. See C. E. Martinez, R. A. Applegate, H. C. Howland, S. D. Klyce, M. B. McDonald, and J. P. Medina, xe2x80x9cChanges in corneal aberration structure after photorefractive keratectomy,xe2x80x9d Invest. Ophthalmol. Visual Sci. Suppl. 37, 933 (1996). Second, by improving schematic model eyes to include higher order aberrations, these new models can provide insight into how the various elements of the eye optical system correlate to affect visual performance. For example, there is a general consensus that the negative asphericity of the normal cornea contributes a negative aberration content. The negative aberration is compensated by a positive aberration contribution from the gradient index nature of the lens. See H. Liou and N. A. Brennan, xe2x80x9cAnatomically accurate, finite model eye for optical modeling,xe2x80x9d J. Opt. Soc. Am. A, Vol. 14, 1684-1695 (1997). The convergence of work on modeling the human optical system with more accurate mathematical descriptions for refraction correction has led to the development of advanced ablation profile algorithms that treat the cornea as the first aspheric element in an optical system. See J. Schwiegerling and R. W. Snyder, xe2x80x9cCustom photorefractive keratectomy ablations for the correction of spherical and cylindrical refractive error and higher-order aberration,xe2x80x9d J. Opt. Soc. Am. A, Vol. 15, No. 9, 2572-2579 (1998).
In view of the recent work on viewing the cornea as an element of a larger optical system, and basing corneal correction with respect to this relation, it is desirable to provide a system and method for laser ablation or reshaping which will provide superior optical results.
It is therefore an object of the invention to provide a system and method for laser ablation or reshaping of the eye which considers higher order aberrations of the eye.
It is another object of the invention to provide a laser ablation or reshaping system and method which treats a broad area of the cornea at one time.
It is a further object of the invention to provide a laser ablation or reshaping system and method more flexible than the broadbeam approach of the prior art.
It is an additional object of the invention to provide a system and method which is capable of directing the laser beam in an asymmetric pattern over the cornea to more particularly correct corneal shape.
It is also an object of the invention to provide a laser ablation or reshaping system and method which can be operated and performed relatively quickly.
It is still another object of the invention to provide a laser ablation or reshaping system and method which is inherently safe from a treatment interruption standpoint.
It is still a further object of the invention to provide a laser ablation or reshaping system and method which does not require advanced high speed tracking of the eye as required in the scanning spot approach.
It is yet another object of the system to provide a laser ablation or reshaping system and method which will significantly reduce the corneal surface roughness seen in other approaches.
In accord with these objects, which will be discussed in detail below, a system and method for performing corneal ablation or reshaping with a laser in order to correct aberrations in the optical system of the eye utilizes a wavefront sensor which defines a wavefront correction for the eye and then, based upon that defined wavefront correction, drives a digital micromirror device (DMD) which modulates a laser beam to the eye to perform the correction. As the DMD is a two dimensional (2-D) array of individually controlled mirrors, and the wavefront sensor analysis can provide a sequence of 2-D arrays of values, the combination of the two produces a method for correcting the corneal surface. The system may be operated in either of two manners to achieve optimum refractive corrections: (1) off-line measurement of the eye optical system via the wavefront sensor followed by DMD-based laser refractive surgery, or (2) real-time measurement of the eye optical system via the wavefront sensor which directs a DMD-based laser refractive surgery system.
With this system and method, the eye surface may be corrected in a manner superior to prior art systems and methods. Wavefront sensing provides an overall refractive analysis of the entire eye optical system, e.g., taking into account the cornea, the lens, the vitreous and the retina. The result of a wavefront sensor analysis yields a waveform model which represents a nearly perfect refraction correction. This provides a superior analysis of the eye relative to the current topography systems which only analyze the cornea. In addition, the use of a DMD to then correct the eye in accord with the wavefront model provides a system which has none of the limitations associated with prior art broadbeam systems and scanning spot systems.
Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures.